... bet lost in fire is a disappointment (7)
I believe the answer is:
setback
'disappointment' is the definition.
(I've seen this before)
'bet lost in fire' is the wordplay.
'lost' is an anagram indicator.
'in' is an insertion indicator.
'fire' becomes 'sack' (dismiss someone from their job).
'bet' is an anagram of 'etb'.
'etb' going within 'sack' is 'SETBACK'.
'is a' is the link.
(Other definitions for setback that I've seen before include "Reversal of fortune" , "Check" , "Problem" , "disappointment" , "unfortunate event" .)